Imaging apparatus

ABSTRACT

Provided are an imaging apparatus and a radiation detecting apparatus comprising a photoelectric conversion layer for converting an incident light into a charge, an electrode layer formed on the photoelectric conversion layer, first and second protective layers formed on the electrode layer, and a transparent electrode disposed between the electrode layer and the first protective layer, wherein a relation of n c1 −n c2 ≦1.5 is met, where n c1  and n c2  are respectively refractive indices of the first and second protective layers.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to imaging apparatus such as lightdetecting apparatus, radiation detecting apparatus, etc. used in medicaldiagnostic imaging apparatus, nondestructive inspection apparatus,analyzing apparatus using radiation, and so on.

[0003] 2. Related Background Art

[0004]FIG. 8 shows an example of an equivalent circuit diagram of animaging apparatus applied to the radiation detecting apparatus, and FIG.9 a plan view thereof. In FIGS. 8 and 9, P11 to P44 designatephotoelectric conversion elements, and T11 to T44 TFTs. Thephotoelectric conversion elements are connected to common bias lines Vs1to Vs4, and a constant bias is applied to them. A gate electrode of eachTFT is connected to a common gate line Vg1 to Vg4. Each gate line isconnected to a gate drive device and on/off of the TFTs is controlled bydrive pulses from the gate drive device. A source or drain electrode ofeach TFT is connected to a common signal line Sig1 to Sig4 and thesignal lines Sig1 to Sig4 are connected to a read device.

[0005] X-rays irradiated toward an object are attenuated and transmittedby the object, the transmitted X-rays are converted into visible lightin a phosphor layer, and this visible light enters the photoelectricconversion elements to generate charges in the respective photoelectricconversion elements. The charges are transferred through the TFTs intothe signal lines by gate drive pulses applied by the gate drive deviceto be read by the read device. Thereafter, the charges generated in thephotoelectric conversion elements are removed by the common bias linesVs1 to Vs4.

[0006] A typical example of the conventional radiation detectingapparatus of this type is a radiation detecting device in which theforegoing phosphor layer is bonded to the imaging apparatus of MIS-TFTstructure comprised of MIS photoelectric conversion elements andswitching TFTs.

[0007]FIG. 10 shows an example of a schematic sectional view of thedevice. Numeral 10 denotes a photoelectric conversion element and 20 aTFT. Numeral 11 designates a lower electrode of the photoelectricconversion element; 12 insulating layers; 15 a bias line for applying abias to the photoelectric conversion element 10; 16 a photoelectricconversion layer of the photoelectric conversion element 10 and asemiconductor layer of the TFT 20; 17 a wire formed on the semiconductorlayer 16 and electrode layers for establishment of ohmic contact of thesemiconductor layer 16; 21 a gate electrode of the TFT 20; 22 source anddrain electrodes of the TFT 20; 30 a phosphor layer for conversion ofincoming radiation into visible light; 31 an adhesive layer for adhesionof the phosphor layer 30; 32 a mounting protective layer; and 36 amoisture-resistant protective layer. The radiation is incident fromabove in FIG. 10 to be converted into visible light by the phosphor, andthe visible light enters the MIS photoelectric conversion element to beconverted into a charge to be stored.

[0008] In the radiation imaging apparatus of this structure, there wereincreasing demands for achievement of higher sensitivity for the purposeof reducing radiation doses and other purposes, while the incomingvisible light was reflected by the protective films and others, so as tocause optical losses, posing a significant issue in the achievement ofhigher sensitivity. Particularly, in the case where there are provided aplurality of protective films having their respective separatefunctions, the foregoing issue can be serious in particular.

[0009] An object of the present invention is, therefore, to provideimaging apparatus and radiation detecting apparatus with highsensitivity on the basis of improvement in a configuration of protectivefilms and others on the photoelectric conversion element to reduce thereflection caused by the films above the photoelectric conversion layer,in order to guide the light emission from the phosphor into thephotoelectric conversion element efficiently.

[0010] In order to achieve the above object, an imaging apparatusaccording to the present invention is an imaging apparatus comprising aphotoelectric conversion layer for converting incident light intocharge, on an insulating substrate, an electrode layer formed on thephotoelectric conversion layer, and a protective layer formed on theelectrode layer, wherein relations of n_(a)−n_(b)≦1.5 andn_(b)−n_(c)≦1.5 are met where n_(a) is a refractive index of thephotoelectric conversion layer, n_(b) a refractive index of theelectrode layer, and n_(c) a refractive index of the protective layer.

[0011] Another imaging apparatus according to the present invention isan imaging apparatus comprising a photoelectric conversion layer forconverting incident light into charge, on an insulating substrate, anelectrode layer formed on the photoelectric conversion layer, and aplurality of protective layers formed on the electrode layer, whereinrelations of n_(a)−n_(b)≦1.5 and n_(b)−n_(c1)≦1.5 and n_(c1)−n_(c2)≦1.5,. . . , and n_(c1)−n_(ci+1)≦1.5 are met where n_(a) is a refractiveindex of the photoelectric conversion layer, n_(b) a refractive index ofthe electrode layer, and n_(c1), n_(c2), . . . , n_(c1) and n_(ci+1)(i=1, 2, 3 . . . ) are refractive indices of the protective layers inorder from the side adjacent to the electrode layer.

[0012] The details will be described in the embodiments of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013]FIG. 1 is a graph for explaining the principle of the presentinvention;

[0014]FIG. 2 is a sectional view of a pixel in the first embodiment ofthe present invention;

[0015]FIG. 3 is a table presenting conditions of refractive indices,absorptivities, and film thicknesses in the first embodiment of thepresent invention and conditions of refractive indices, absorptivities,and film thicknesses in a conventional example;

[0016]FIG. 4 is a graph showing change in the ratio of quantity of lightincident into a semiconductor layer 16 to quantity of light incidentinto a mounting protective layer 32 against change in film thickness ofa moisture-resistant protective layer 36 under Conditions (1) to (3) inFIG. 3;

[0017]FIG. 5 is a sectional view of a pixel in the second embodiment ofthe present invention;

[0018]FIG. 6 is a table showing conditions of refractive indices,absorptivities, and film thicknesses applied in comparison of the secondembodiment of the present invention;

[0019]FIG. 7 is a graph showing change in the ratio of quantity of lightincident into the semiconductor layer 16 to quantity of light incidentinto the mounting protective layer 32 against change in film thicknessof the moisture-resistant protective layer 36 under Conditions (1) and(2) in FIG. 6;

[0020]FIG. 8 is a diagram showing an equivalent circuit of an imagingapparatus;

[0021]FIG. 9 is a plan view of the imaging apparatus;

[0022]FIG. 10 is an example showing a sectional view of a pixel in theimaging apparatus;

[0023]FIG. 11 is a sectional view of a pixel in the imaging apparatus ofthe third embodiment according to the present invention;

[0024]FIG. 12 is a table presenting a condition of refractive indices,absorptivities, and film thicknesses in the third embodiment; and

[0025]FIG. 13 is a graph showing change in the ratio of quantity oflight incident into the semiconductor layer 16 to quantity of lightincident into the mounting protective layer 32 against change in filmthickness of the moisture-resistant protective layer 36 under thecondition in FIG. 12.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0026] The embodiments of the present invention will be described belowin detail with reference to the accompanying drawings. An example of anequivalent circuit and a plan view of an imaging apparatus in thepresent embodiment will be described referring to FIGS. 8 and 9.

[0027] In the following, let us assume, as an example of a basicconfiguration of the imaging apparatus, a configuration in which aphosphor layer is provided as a wavelength conversion element and inwhich a photoelectric conversion layer, an electrode layer, and aprotective layer are stacked in the order named from the substrate side,as shown in FIG. 10, and let us explain the relation between refractiveindices and quantities of incident light in the stacked films in thisconfiguration. Let us suppose that the refractive index n_(a) of thephotoelectric conversion layer is equal to 4.4, the refractive indexn_(b) of the electrode layer 3.8, and the refractive index n_(c) of theprotective layer variable. Let us also suppose that the absorptioncoefficient k_(a) of the photoelectric conversion layer is equal to0.40, the absorption coefficient k_(b) of the electrode layer 0.15, andthe absorption coefficient k_(c) of the protective layer 0.00. Let d_(a)be the film thickness of the photoelectric conversion layer, the filmthickness of the electrode layer be d_(b)=60 nm, and the film thicknessof the protective layer be constant.

[0028] Under the above conditions, the difference of the refractiveindex of the protective layer from the refractive index of the electrodelayer, Δn=n_(b)−n_(c), is plotted on the horizontal axis, while theratio of quantity of light incident into the photoelectric conversionlayer to quantity of light incident into the protective layer is plottedon the vertical axis, thereby obtaining the result of FIG. 1. Inpractical imaging apparatus, the output thereof has variation ofapproximately ±5% among apparatus and in the apparatus surface becauseof variation of film thickness of the photoelectric conversion layer andothers.

[0029] Namely, in FIG. 1 the loss of quantity of incident light isdesirably within 10% relative to the case of the difference between therefractive indices being 0, and thus the difference Δn of the refractiveindex of the protective layer from the refractive index of the electrodelayer is desirably not more than 1.5. The difference of the refractiveindex of the electrode layer from the refractive index of thephotoelectric conversion layer is 0.6 in FIG. 1, and is thus not morethan 1.5.

[0030] In the imaging apparatus having the above-stated layerconfiguration, as described above, the loss is small in the quantity oflight incident into the photoelectric conversion layer and can be within10% when the relations of n_(a)−n_(b)≦1.5 and n_(b)−n_(c)≦1.5 are metwhere n_(a) indicates the refractive index of the photoelectricconversion layer, n_(b) the refractive index of the electrode layer, andn_(c) the refractive index of the protective layer.

[0031] Likewise, in the case of a plurality of protective layers beingformed, the loss can also be small in the quantity of light incidentinto the photoelectric conversion layer when the relations ofn_(a)−n_(b)≦1.5 and n_(b)−n_(c)≦1.5 and n_(c1)−n_(c2)≦1.5, . . . andn_(ci)−n_(ci+1)≦1.5 are met where the refractive index of thephotoelectric conversion layer is n_(a), the refractive index of theelectrode layer is n_(b), and the refractive indices of the protectivelayers are n_(ci), n_(c2), . . . , n_(ci), and n_(ci+1) (i=1, 2, 3, . .. ) in order from the side adjacent to the electrode layer.

Embodiment 1

[0032] The first embodiment of the present invention will be describedbelow with reference to the drawings. FIG. 2 shows a sectional view of apixel in the present embodiment, in which numeral 10 designates aphotoelectric conversion element and 20 a TFT. In the photoelectricconversion element 10 and TFT 20, electrode layers 17 (n⁺ layers herein)for establishment of ohmic contact of P or B-doped amorphous silicon arelaid on semiconductor layers 16 of amorphous silicon, and a surfacestabilizing protective layer 37 for the TFT, a protective layer 36 formoisture resistance, and a protective layer 32 for mounting are laid asprotective layers in the order named on the electrode layers 17 so as tocover the photoelectric conversion element 10 and the TFT 20. In thepresent embodiment, SiN-2 is used for the TFT surface stabilizingprotective layer 37, SiN-1 for the moisture-resistant protective layer36, and PI (polyimide) for the mounting protective layer 32. Namely,three functionally separate protective layers are formed.

[0033] A photosensor consists of a plurality of pixels in the layerstructure as described above, and, for example, a phosphor layer 30 isbonded as a wavelength conversion element for converting radiation suchas X-rays or the like into light such as visible light or the like,through an adhesive layer 31 to the photosensor.

[0034] In this configuration, let n_(a) and n_(b) be the refractiveindices of the semiconductor layers 16 and the n⁺ layers 17,respectively, n_(c) be the refractive index of the moisture-resistantprotective layer, k_(a), k_(b) and k_(c) be the absorptivities of therespective layers, and d_(a), d_(b) and d_(c) be the film thicknesses ofthe respective layers.

[0035]FIG. 4 shows the results obtained where the refractive indices,absorptivities, and film thicknesses of the respective films for thelight with the wavelength of 550 nm near an emission peak of GOS used asthe phosphor layer 30 are in the relations of (1), (2) and (3) in FIG.3. FIG. 4 shows the results of comparison among the conditions of (1),(2) and (3) in FIG. 3, in which the horizontal axis represents the filmthickness of the moisture-resistant protective layer and the verticalaxis the ratio of quantity of light incident into the semiconductorlayer 16 to quantity of light incident into the mounting protectivelayer 32.

[0036] The condition (1) represents the layer structure shown in FIG.10, and SiN-1 with the refractive index of 1.9 is used for themoisture-resistant protective layer 36. The condition (2) represents theresult similarly using the radiation detecting apparatus of the layerstructure shown in FIG. 10, but SiN-2 with the refractive index of 2.4,by which the index difference from the n⁺ layers 17 becomes not morethan 1.5, is used for the moisture-resistant protective layer 36.

[0037] The condition (3) represents the result in the case of the layerstructure shown in FIG. 2, in which a film of SiN-2 as the TFT surfacestabilizing protective layer 37 and a film of SiN-1 as themoisture-resistant protective layer 36 are formed on the photoelectricconversion element 10 and the TFT 20.

[0038] In all the conditions herein, the refractive indices are in therelation of n_(a)>n_(b)>n_(c), but in the condition (1) the indexdifference between the n⁺ layers 17 and SiN-1 as the moisture-resistantprotective layer 36 is as large as 1.9, thus producing a large loss inthe quantity of incident light due to the reflection at the interfacebetween them.

[0039] In contrast to it, in the condition (2) the index differencebetween the n⁺ layers 17 and SiN-2 as the moisture-resistant protectivelayer 36 is 1.4, so that the reflection is reduced at the interface.Therefore, the quantity of incident light is as large as approximately80%, depending upon the film thickness of the protective layer. However,the light is absorbed in SiN-2 with the refractive index of 2.4, and theloss in the quantity of incident light can be greater in the filmthickness capable of functioning as a protective layer for moistureresistance than in the condition (1). Accordingly, this configuration isalso effective with sufficient quantity of incident light in the casewherein in FIG. 4 the film thickness is within 100 nm, the filmthickness distribution can be controlled with certain degree ofaccuracy, and the layer of that film thickness can fully provide thefunction as a protective film. In this configuration, if the protectivefilm is implemented as a thin film having the refractive index ofapproximately 2.4 and demonstrating little absorption for the lightconverted by the phosphor layer of the wavelength conversion element,particularly, having the absorption coefficient of 0, the change will besmall in the quantity of incident light against change of filmthickness, so that the effect can be further enhanced.

[0040] In contrast to it, the condition (3) represents a configurationin which the index differences from the n⁺ layers 17 and from SiN-1 asthe moisture-resistant protective layer 36 are made smaller by SiN-2 asthe surface stabilizing protective layer 37 and in which the differencefrom the mounting protective layer is further decreased, so-as to meetthe relations of n_(a)−n_(b)≦1.5 and n_(b)−n_(c)≦1.5 andn_(c1)−n_(c2)≦1.5, whereby the loss can be reduced in the quantity ofincident light. In addition, it also becomes feasible to keep small therate of change in the quantity of incident light against film thicknessdistribution of the protective films.

[0041] Specifically, as shown in FIG. 4, the minimum quantity ofincident light, which varies between minima and maxima depending uponthe film thicknesses, was increased by 8%, from 65% in (1) to 73% in(3). When films with a large index difference are stacked adjacent toeach other, there are cases where the variation in film thickness of themoisture-resistant protective layer 36 greatly affects the quantity ofincident light, as in (1). Therefore, the effect of reducing thevariation of sensitivity can also be presented, particularly, bydecreasing the index difference between adjacent layers, as in (3).

Embodiment 2

[0042] The second embodiment of the present invention will be describedbelow with reference to the drawings.

[0043]FIG. 5 shows a sectional view of a pixel in the presentembodiment. In the photoelectric conversion element 10 and the TFT 20,the electrode layers 17 (n⁺ layers herein) of ohmic contact layers arelaid on the semiconductor layers 16, and a TFT surface stabilizingprotective layer 37, a planarization film 39, a moisture-resistantprotective layer 36, and a mounting protective layer 32 as protectivelayers are stacked in the order named on the electrode layers 17.

[0044] SiN-2 is used for the TFT surface stabilizing protective layer37, BCB (benzocyclobutene) for the planarization film, SiN-1 for themoisture-resistant protective layer 36, and PI (polyimide) for themounting protective layer 32.

[0045] A photosensor consists of a plurality of pixels having the layerstructure as described above, and a wavelength conversion element forconverting radiation such as X-rays or the like into light such asvisible light or the like, e.g., a phosphor layer 30 is bonded throughan adhesive layer 31 to the photosensor.

[0046] In this structure, let n_(a) and n_(b) be the refractive indicesof the semiconductor layers 16 and the n⁺ layers 17, respectively,n_(c1), n_(c2), . . . , n_(c1) be the refractive indices of therespective protective layers in the order from the refractive index ofthe layer on the electrode side, k_(a), k_(b), k_(c1), k_(c2), . . . ,k_(c1) be the absorptivities of the respective protective layers in thesame order, and d_(a), d_(b), d_(c1), d_(c2), . . . , d_(ci) be the filmthicknesses of the respective protective layers in the same order.

[0047]FIG. 7 shows the results obtained where the refractive indices,absorptivities, and film thicknesses of the respective films for thelight with the wavelength of 550 nm near an emission peak of CsI used asthe phosphor layer 30 are those in the conditions (1) and (2) in FIG. 6.FIG. 7 shows the results in the cases (1) and (2) therein, in which thehorizontal axis represents the film thickness of the moisture-resistantprotective layer and the vertical axis the ratio of quantity of lightincident into the semiconductor layer to quantity of light incident intothe mounting protective layer.

[0048] (1) indicates the result in the case of a configuration whereinthe planarization film 39 with the refractive index of 1.6 is formed onthe N⁺ layers 17 and the moisture-resistant protective layer 36 with therefractive index of 1.9 is formed so as to cover the planarization film39. Namely, this is an example wherein the sensor is constructed in theconfiguration without the TFT surface stabilizing protective layer 37 inFIG. 5.

[0049] (2) represents the result in the structure shown in FIG. 5, whichis an example of a configuration wherein SiN-2 with the refractive indexof 2.4 is formed as the TFT-surface stabilizing protective layer 37 onthe N⁺ layers 17, the planarization film 39 is formed so as to cover it,and SiN-1 is further formed as the moisture-resistant protective layer36 thereon.

[0050] As apparent from FIG. 7, since (1) is the condition that theindex difference between the N⁺ layers 17 and BCB of the planarizationfilm 39 is as large as 2.2, the loss is also very large in the quantityof incident light due to the reflection at the interface between them.

[0051] In contrast to it, (2) is the condition that SiN-2 is formed asthe TFT surface stabilizing protective layer 37 to achieve the indexdifference of 1.2 between the refractive indices of BCB of theplanarization layer 39 and SiN-2 and the index difference of 1.4 betweenthe refractive indices of the N⁺ layers 17 and SiN-2, whereby thereflection is reduced at the interfaces between them.

[0052] Namely, the loss in the quantity of incident light can bereduced, because the relations of n_(a)−n_(b)≦1.5 and n_(b)−n_(c)≦1.5and n_(c1)−n_(c2)≦1.5 and n_(c2)−n_(c3)≦1.5 are met.

[0053] Specifically, the minimum quantity of the incident light, whichvaries between minima and maxima depending upon the film thicknesses,was increased by 14%, from 61% in (1) to 75% in (2). When films with alarge index difference are adjacent to each other, the variation in thefilm thickness of the moisture-resistant protective layer can greatlyaffect the quantity of incident light, as in (1). Therefore, theconfiguration of the present embodiment can also achieve the effect ofreducing the variation of sensitivity.

Embodiment 3

[0054] The third embodiment of the present invention will be describedbelow with reference to the drawings.

[0055]FIG. 11 shows a sectional view of a pixel in the presentembodiment. In the photoelectric conversion element 10 and the TFT 20,the electrode layers 17 (N⁺ layers herein) of ohmic contact layers arelaid on the semiconductor layers 16, and the present embodiment isdifferent from the other embodiments in that a transparent conductivelayer, ITO 40 in the present embodiment, is formed on the electrodelayers. When ITO is further provided on the bias line as in this case,it becomes feasible to decrease the film thickness of the N⁺ layer. ThisITO is present on the sensor element, but it may or may not be presenton the source and drain electrodes of the TFT.

[0056] A photosensor consists of a plurality of pixels having the layerstructure as described above, and a wavelength conversion element forconverting the radiation such as X-rays or the like into light such asvisible light or the like, e.g., the phosphor layer 30, is bondedthrough an adhesive layer 31 to the photosensor.

[0057] In this structure, let n_(a) andn_(b be the refractive indices of the semiconductor layers 16 and the N)⁺ layers 17, respectively, the refractive index of ITO n_(c1)=1.9, andthe absorption coefficient thereof k_(c1)=0.00.

[0058]FIG. 13 shows the result obtained where the refractive indices,absorptivities, and film thicknesses of the respective films for thelight with the wavelength of 550 nm near the emission peak of CsI usedas the phosphor layer 30 are those in the condition of FIG. 12. In FIG.13, the horizontal axis represents the film thickness of themoisture-resistant protective layer and the vertical axis the ratio ofquantity of light incident into the semiconductor layer to quantity oflight incident into the mounting protective layer.

[0059] In the present embodiment, as described above, the transparentelectrode layer of ITO or the like is interposed between the protectivefilm and the electrode layer (N⁺ layer) whereby it becomes feasible toincrease the quantity of incident light into the sensor element, ascompared with Embodiments 1 and 2, and to keep small the change in thequantity of incident light against change in the film thickness of theprotective layer.

1. to
 17. (Cancelled)
 18. An imaging apparatus comprising: aphotoelectric conversion layer for converting an incident light into acharge; an electrode layer formed on the photoelectric conversion layer;first and second protective layers formed on the electrode layer; and atransparent electrode disposed between the electrode layer and the firstprotective layer, wherein a relation of n_(c1)−n_(c2)≦1.5 is met, wheren_(cl) and n_(c2) are respectively refractive indices of the first andsecond protective layers.
 19. The imaging apparatus according to claim18, wherein said photoelectric conversion layer is amorphous silicon.20. The imaging apparatus according to claim 18, wherein said electrodelayer is P or B-doped amorphous silicon.
 21. The imaging apparatusaccording to claim 18, wherein said first and second protective layersare silicon nitride.
 22. The imaging apparatus according to claim 18,wherein said first protective layer is silicon nitride, and said secondprotective layer is polyimide.
 23. The imaging apparatus according toclaim 18, wherein the electrode layer has a film thickness of 15-30 nm.24. The imaging apparatus according to claim 18, further comprising awavelength conversion element.
 25. The imaging apparatus according toclaim 24, wherein the wavelength conversion element converts light intoone which has a peak wavelength of approximately 550 nm.